1. Field of the Invention
The present invention relates laser peening, and more specifically, it relates to techniques for applying an ablative layer that will not create pitting when performing laser peening.
2. Description of Related Art
The use of an ablation layer in laser peening is well known. See, e.g., U.S. Pat. No. 4,937,421 and C. S. Montross et al., which are briefly discussed below.
In U.S. Pat. No. 4,937,421, titled “Laser peening system and method,” a laser peening apparatus and method for peening a workpiece utilizing a laser beam is described. The system includes a foil aligned with a surface of the workpiece to be peened and lasing the aligned foil surface. The foil absorbs energy from the beam and a portion of the foil vaporizes, which creates a hot plasma within the foil. The plasma creates a shock wave which passes through the foil and peens the workpiece surface.
In a paper by C. S. Montross et al., titled: “The Influence of Coating on Subsurface Mechanical Properties of Laser Peened 2011-T3 Aluminum”, Journal of Material Science 36 (2001) 1801-1807, an ablative, sacrificial coating such as paint or metal foil is discussed for use to protect the aluminum component from surface melting by the laser pulse, which adversely affects fatigue life. This paper, using nano-indentation, analyzes the effect of the paint and foil coatings on the shock wave propagation into the aluminum specimen and the resulting change in mechanical properties versus depth Near the surface, hardness was found to be increased by the laser peening, however this process decreased the measured elastic modulus. The laser pulse energy density and properties of the foil including its adhesion to the aluminum alloy were found to influence the change in surface mechanical properties.
In the process of laser peening, a high power laser is made incident onto a metal surface, ablating a thin surface layer, creating a plasma and a consequent intense shock. This intense shock plastically strains the material and results in a compressive layer of residual stress in the surface. To avoid contact of the hot plasma with the metal surface, a layer or layers of material are applied to the substrate surface to act as a source of ablation material and to provide insulation from the plasma's heat. This plasma is the source of a shock wave that forms and consequently peens the material. Originally paint was used as the ablation layer but paint does not have sufficient tensile strength to keep from locally shearing when the shock locally compresses the surface. It also fractures and debonds when the shock ends and the surface rebounds. Tapes with adhesive backing have proved to be a better ablation material. They are readily applied, stick to the surface and have sufficient tensile strength to allow continuous side-by-side peening without in general shearing or debonding. However it has been realized that in applying the tape to surfaces, small pockets of air, often as small as 20 μm in semi-spherical size can be trapped under the tape and in contact with the metal surface. When these pockets are compressed by the shock wave they heat in an adiabatic manner and transmit this heat to the metal. For metals that melt at low temperature, such as aluminum, this transmitted heat can be sufficient to locally melt the metal and create a small pit of molten material that subsequently solidifies as a solid crater. This pit is undesirable as it solidifies in a tensile state forming a stress riser on the metal surface and potentially reduces the fatigue lifetime and resistance to corrosion of the peened sample.